- The Main Status Byte (STB i), as read with
"STB i,?", keeps
accumulating further bits as they get set.
- The Main Status Byte (STB I), as read with the
Serial Poll, only
shows the bits that were set at the time the RQS
was set. Any bits
that should get set due to subsequent events are not taken into
account (but are remembered), until a Serial Poll reads STB 1 and
clears it; at this point in time, any remembered bit is set in STB 1
and may again generate an SRQ. The principle in Serial Poll is that
SRQ may be generated by one event at a time.
Example:
MASK 1,48
Sets the mask of the Main Status Byte (STB i) to the
binary value 00110000, allowing the ERROR bit or the
OPERATION COMPLETE bit to generate an RQS bit, i.e. to
generate a Service Request.
MASK 5,4
Sets the mask of the OPERATION COMPLETE byte (STB 5)
the binary value 00000100, allowing the propagation of the
"calibration done" bit to the OPERATION COMPLETE bit of
STB i.
MASK 6,1
Sets the mask of the ERROR byte (STB 6) to a non-zero
value, allowing the propagation of errors to the ERROR bit
of STB i.
CAL
Instructs the 9400A to calibrate itself. After
approximately 500 msec, the 9400A sets the "calibration
done" bit in the OPERATION COMPLETE byte (STB 5). Since
this bit is demasked, it is propagated to the OPERATION
COMPLETE bit of STB I. And because this bit is also
demasked, it sets the RQS bit and generates a Service
Request interrupt on the GPIB bus.
TRIG LEVEL
Instead of responding to the Service Request, the host
computer sends this illegal command. It generates a
semantic error and sets the ERROR byte (STB 6) to the
value 40. Since this byte is demasked, the ERROR bit in
STB 1 is also set, and because this bit is also demasked,
it may set the RQS bit. However, the RQS bit is already
set.
At this point, the host computer may choose to respond to the Service
Request in either of the following ways:
7-53
Remote Operations
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